A pair of annular bead cores embedded and arranged in bead portions of a pneumatic tire holds a carcass ply, which includes a body and a folded portion connected to the body, in the bead portions and prevents a carcass ply from being drawn back from a bead portion against the action caused by an internal pressure of the filled tire or applied load to the tire. The body toroidally extends between the bead portions, and the folded portion is folded around the bead core.
The bead core is generally formed by coiling one or more steel cord or the like in a tire width direction and repeatedly coiling the cord in a tire radial direction such that given rows and given columns of the cords are arranged in a cross section in the tire width direction. In the cross section in a tire width direction, the bead core has a polygonal (e.g. a hexagonal) shape and includes at least one corner on the side of the body of the carcass ply.
The tire including the bead core having a polygonal shape cross section has a problem of durability of the carcass ply. That is, every time the tire rotates and is loaded and thus tension in such a direction that the carcass ply is drawn back is applied to the carcass ply, the body of the carcass ply is repeatedly pressed to the corner of the bead core on the side of the body. In particular, when the tire is used with unexpectedly high internal pressure or unexpectedly heavy load, press force to the corner of the bead core is increased to concentrate the stress of the carcass ply and thus the durability of the carcass ply is more degraded.
Patent Literature 1 discloses a pneumatic tire having a layer of rubber-coated and lined organic fiber cords sandwiched between a bead core and a carcass ply, the cords are tilted to a tire circumferential direction and coiled over the bead core. According to the tire, the distance between the body of the carcass ply and the corner of the bead core is secured with the organic fiber cord in the layer of organic fiber cords and coating rubber which coats the cords. The secured distance therebetween, in particular the thickness of the coating rubber, contributes to reduce the stress concentration at the carcass ply pressed by the corner of the bead core and to partially prevent degradation of durability of the carcass ply when the tire rotates and is loaded.